Where several low-speed data streams (tributaries) are to be sent on one high-speed data stream (carrier), there is a need for a multiplexer. A multiplexer serves the purpose, at a sending end, of arranging data from a number of tributaries into a single, complex signal that modifies a carrier wave. At the receiving end, the individual tributaries are regenerated into separate signals.
When data is transmitted using a bandwidth normally used for one telephone voice channel, a standard digital transmission rate may be used, called DS-0. Generically, digital signal X (DS-X) is a term for a series of standard digital transmission rate or levels based on DS-0, which has a transmission rate of 64 Kbps. Both the North American T-carrier system and the European E-carrier systems of transmission operate using the DS series as a base multiple. DS-0 is the base for the digital signal X series. DS-3, the signal in the T-3 carrier, carries a multiple of 28 DS-1 signals or 672 DS-0s and has a net rate of 44.736 Mbps.
Streams of digital data in communication systems are often broken up into “octets” which are sequences of eight bits, which may also be called bytes. The North American standard for digital networks that employ optical fiber is called synchronous optical network (SONET) while the European standard is called synchronous digital hierarchy (SDH). Both use octet multiplexing to create a higher-speed stream from lower-speed tributary signals.
In octet multiplexing successive time slots on the carrier are allocated to octets from different tributaries. In a SONET based transmission system, the output of the multiplexer may be a “Synchronous Transport Signal Level 1(STS-1)” signal with a basic bit rate of 51.84 Mbps. A standard STS-1 frame consists of 6480 bits or 810 octets and is 125 μs in duration. The time slots containing octets (or bytes) may be seen to be a frame organized in rows and columns, for STS-1 specifically, nine rows and 90 columns. Not all of an STS-1 frame is payload (corresponding to data from tributaries), as 36 octets (three columns) are reserved for overhead information used for such purposes as frame identification and monitoring of errors. The other 87 columns comprise a synchronous payload capacity. Into the capacity will be mapped an 87 column synchronous payload envelope (SPE). Typically, the SPE consists of one column of path overhead 86 columns of payload.
At a sending end, octets from a tributary are mapped into an SPE. Typically the clock of a mapping processor at the sending end is synchronous with the clock of the outgoing transport signal. Note that once an SPE has been mapped into an STS-1 frame, multiple STS-1 frames may be multiplexed together into a higher order signal, such as STS-3. At a receiving end, octets from a specific tributary are de-mapped from an SPE, received at a corresponding elastic store or FIFO (first in first out) buffer and then output from the buffer with timing from a clock local to the receiver. Typically the clock of de-mapping processor at the receiver is synchronous with the clock of the incoming transport signal. However, there may be transitions in the path between the sending end and the receiving end whereat one or more SPE's within STS-1 frames are passed to a new transport (STS-N). Unfortunately, the clocks at these transitions may not be synchronous. As a result of this asynchrony, the phase of a clock at the receiving end may be receding or advancing relative to the corresponding clock at the sending end.
One challenge is to regenerate the sending end clock from the de-mapped stream of data while minimizing jitter and wander (where wander may be considered a low frequency form of jitter). Methods for mapping one rate or format into another as well known. Specifications of methods for mapping the common asynchronous transmission formats (DS0, DS1, DS2, DS3, etc) into SONET are described in detail in “Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria, Technical Reference TR-NWT-000253, Issue 2 (Dec. 1991), Bell Communications Research, Inc.” (the contents of which are incorporated herein by reference). Similar mapping methods are defined for the European Telecommunications Standardization Institute (ETSI) hierarchy mapping into SDH. Also, optical transmission equipment may map one proprietary format into another. For example, Nortel FD-565 (from Nortel Networks Corporation of Montreal, Canada) can carry a FD-135 proprietary format as well as the DS-3 standard format.
Specifically, for mapping DS-3 into STS-1 a mapping technique must be used that compensates for timing variations due to the asynchronous nature of the DS-3 signal. Usually an asynchronous stuffing bit is added in each SPE row while fixed stuffing bits are added to fill the STS-1 frame.
These methods are each optimized for the mapping of particular formats, and cannot be used to map rates that vary significantly from the standard. These mappings are also each precisely tuned for the particular format and bit-rate that is being mapped, plus or minus a tolerance such as 20 parts per million on the bit rate. A signal that has a bit rate even 1% different than that of a DS-3 may not be transported, using these methods, within SONET. A different hardware unit is generally required to perform the mapping of each format of signal. Thus the standards have allowed transportation of a very specific set of signals with format specific hardware.
When known mapping techniques are adapted to achieve low amounts of jitter at the far end, given that a clock is regenerated from the de-mapped stream of data, the jitter may approach the maximum spacing that occurs between data chunks (e.g., bits, bytes, etc.).
Rather than mapping to minimize jitter, an alternative solution is to filter the regenerated clock with a very low bandwidth phase locked loop (PLL). This requires a voltage controlled crystal oscillator (VCXO) based analog PLL or a crystal oscillator (XO) based, digitally controlled PLL, each of which introduce cost and complexity issues. Further, the low bandwidth associated with these solutions make addressing frequency steps, that may occur on the tributary signal, challenging. To accommodate this, a large FIFO is required, making implementation slightly more expensive and extending latency.
There remains a need for an efficient method for mapping, using inexpensive hardware, arbitrary signals into SONET (and other formats) such that the signals can be recovered with low timing jitter.